1. Field of the Invention
The present invention relates to a process for producing a magnetic powder. More particularly, it relates to a process for producing a magnetic powder having excellent magnetic characteristics.
2. Description of the Prior Arts
Various magnetic powders used for magnetic recording media such as a magnetic tape have been proposed. Typical magnetic powders include acicular .gamma.-Fe.sub.2 O.sub.3, acicular Fe.sub.3 O.sub.4 or cobalt-doped or cobalt-adsorbed acicular .gamma.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4 or metallic powders such as acicular metallic iron. Recently, the metallic powders have been practically used in view of high density recording. The metallic magnetic powders can be produced by reducing iron oxide in hydrogen gas or an inert gas by carrying out a wet reduction process from an aqeuous solution of an iron salt with or without another salt such as a cobalt salt.
It has been known as a wet reduction process, to produce a magnetic powder as a product by mixing an aqueous solution of a metal salt with an aqueous solution of a reducing agent such as sodium boron hydride in a magnetic field to reduce the metal salt so as to continuously form a metallic magnetic powder and heat-treating the resulting magnetic powder in a non-oxidative atmosphere. In this process, it has been found to cause sintering between metallic particles during the heat-treatment in the non-oxidative atmosphere to cause the deterioration of the magnetic characteristics.
The phenomenon has been studied to find the fact that the melt-bonding of the metallic particles is accelerated during the heat-treatment because a boron component used in the wet reduction process is remained in the magnetic powder.
It has been further found that a magnetic tape prepared by using a metallic magnetic powder having relatively high boron content, has a disadvantage of inferior stability in the condition of high humidity and high temperature such as 50.degree. C. 90 RH to cause variation in storage. The variation in storage should be prevented since it may gradually cause even in the normal condition. The adverse effect of the boron component to the magnetic characteristics has been found by the inventor and has not been previously found.